1. Field of Invention
The present invention is in the field of phase-shift control in optical-hybrid etalons having simultaneous phase- and polarization-diversity properties.
2. Discussion of Prior Art
An unbalanced hybrid produce output waveforms with prescribed phase-shifts from p-branches whereas in a balanced hybrid the outputs are from 2p-branches [1]. For example, p=2 corresponds to a 90.degree.-hybrid having two outputs with quadraturely-phased signals, and p=3 corresponds to a 120.degree.-hybrid having three output signals separated by 120.degree. in phase between ports. Other p-branch versions can also be formed. Balanced hybrids generate both a plus and a minus phase signal such that when the two signals are detected by a balanced-pair photodiode the dc components of the signals can be subtracted thereby minimizing the LO (local oscillator) laser intensity-noise [2] in homodyne receivers. The LO intensity noise in unbalanced hybrids can also be minimized by the use of an auto-balanced photoreceiver [3] because the receiver subtracts a reference beam (as derived from the LO beam) from the signals. Single-channel hybrids could be made insensitive to either phase or polarization fluctuations but not simultaneously to both types of fluctuations. However, simultaneous diversity (or immunity) to both phase and polarization can now be obtained with the recently developed single-channel optical-hybrid etalons [4]. Performance of balanced 90.degree.- and 180.degree.-hybrid etalons have been reported in reference [5].
In some hybrids the phase relationship in the output beams of a hybrid cannot be changed once a hybrid configuration has been selected. For example, in a fiber-optic hybrid the phase-shift is monitored during the drawing/fusing process in order to fix a prescribed phase shift in the output fibers of the coupler. The phase relationship between the output fibers cannot be altered after the fibers have been fused [6]. In bulk-optic hybrids, generally, the optical components are bonded in a miniaturized encapsulated package and thus cannot be easily re-assembled to other phase configurations. Limited phase control is possible in an integrated-optic hybrid [7] by varying the refractive index of the electro-optic waveguide with an applied electric field. In this present invention active phase control in an optical-hybrid etalon will be described.